Referring to FIG. 1, a conventional oxide cathode K comprises:                a cathode emissive coating 1 essentially composed of alkaline-earth oxides;        a metal substrate 2 on which the cathode emissive coating is deposited, which substrate is generally based on nickel and/or a nickel alloy, containing elements for reducing the alkaline-earth oxides, such as magnesium and silicon;        a hollow tubular skirt 3 supporting the substrate; and        a filament 4 placed in the skirt 3, suitable for heating the substrate 2 and the cathode emissive coating 1 under vacuum to a temperature sufficient for this coating to emit electrons.        
The cathode emissive coatings are generally porous, especially because they are generally made by thermal decomposition of carbonates of alkaline-earth elements into oxides of these elements; before decomposition, their thickness is generally between 53 and 95 μm, so as to obtain, after decomposition, an oxide coating having a thickness of between 50 and 90 μm.
The thickness of the substrate 2 is generally between 70 and 150 μm.
The skirt and the substrate may be produced from the same metal piece—the cathode is then referred to as a “one-piece” cathode.
Such cathodes are used in electron guns, especially for cathode-ray tubes; an electron gun conventionally comprises:                a triode, shown in FIG. 1, suitable for forming an electron beam, comprising a cathode K, a first electrode G1 or Wehnelt pierced by a hole 52 forming the base of said electron beam and a second electrode G2 also pierced by a hole for passage of the electrons of the beam; and        means (not shown) for focusing the electron beam coming from the triode, generally comprising a series of electrodes suitable for this purpose.        
The electrode closest to the cathode, or Wehnelt G1, has a plane surface 51 lying parallel to the external surface of the cathode emissive coating 1 and at a distance from this surface of around 50 to 80 μm when the gun is in operation.
The hole 52 in the Wehnelt G1 of the triode may be circular, elliptical or even rectangular; the cathode and the electrode G1 of the triode are positioned so that the center of the cathode emissive coating coincides approximately with the center of the hole 52 in this electrode; two zones are therefore distinguished in the cathode emissive coating:                a central zone 12 corresponding to the zone facing the hole 52 in the electrode G1, and therefore having the same shape as this hole, as indicated by the dotted lines on either side of the axis of symmetry in FIG. 1; and        a peripheral lateral zone 11 corresponding to the entire cathode emissive coating except for the central zone.        
The tube of a conventional color television with a shadow mask comprises in general a triple triode including three cathodes, one per primary color—red, green and blue; for each cathode, the electron beam current is generally around 0.4 to 0.8 mA averaged over time; for maximum luminances, this current may typically be up to 5 mA and the entire emissive surface of the cathode is then used; in the case of circular holes in the electrode G1, for example with a diameter of 0.5 mm, the potentially emissive zone of the cathode is then also circular and has the same diameter.
When the triode is in operation, it is therefore the voltage difference applied between the substrate 2 of the cathode and the Wehnelt G1 which serves to modulate the cathode emission; the triode also includes an electrode G2, further away from the cathode than the electrode G1 along the trajectory of the electrons; when the triode is in operation, the electrode G2 is biased so as to exert on the cathode a positive constant electric field for extracting the electrons, whereas the electrode G1 is biased so as to exert a negative adjustable field; in practice, the central zone 12 corresponds to the maximum extent of the electron-emitting zone that is obtained for the lowest voltage differences between the cathode and the electrode G1 this central, potentially emissive, zone will therefore be called hereafter, and by extension, the “emissive zone”; by analogy, the peripheral zone will therefore be called hereafter the “nonemissive zone”.
The voltage difference between the substrate and the Wehnelt, for which the cathode emission is reduced to zero is usually called the “cut-off voltage”.
Throughout the lifetime of a cathode, it is known that the cut-off voltage drifts, and this degrades the emission performance of the gun or requires an expensive compensation system.
The emission performance of a cathode is moreover expressed in terms of “maximum emission in pulse mode” and in terms of “maximum emission in DC mode”.
The maximum emission in pulse mode corresponds to the maximum surface current density of the cathode emissive coating that can be achieved during a voltage pulse of short duration, of around 10 μs, applied to the grid G1; for cathode emissive coatings whose thickness is between 50 μm and 90 μm, the maximum emission in pulse mode of a cathode is essentially proportional to the thickness of its cathode emissive coating and generally depends little on the porosity of this coating.
The maximum emission in DC mode corresponds to the maximum surface current density of the cathode emissive coating that can be reached during a voltage pulse of long duration, of the order of 10 s, applied to the grid G1; the maximum emission in DC mode of a cathode is essentially proportional to the conductivity of its cathode emissive coating measured through its thickness; this maximum DC emission is therefore essentially inversely proportional to the thickness and proportional to the porosity of this coating.
During the lifetime of a cathode, the porosity and the thickness of its cathode emissive coating decrease by a sintering effect; overall, the conductivity of this coating decreases, thereby causing the maximum emission in DC mode to be lowered.
On the other hand, during the lifetime of a cathode, the maximum emission in pulse mode degrades less than the maximum emission in DC mode.
The cut-off voltage drift stems in particular from the variation in the distance separating the cathode emissive coating of the gate G1 from the electron gun; this distance varies especially because the porous cathode emissive coating shrinks by sintering; this shrinkage is greater the greater the initial thickness and/or the greater the porosity of the coating.
To recapitulate:                a high porosity results in an improvement in the maximum emission in DC mode but a degradation in the cut-off voltage drift; and        a high thickness results in an improvement in the maximum emission in pulse mode but a degradation in the maximum emission in DC mode and in the cut-off voltage drift.        
It may therefore be seen that one is faced with a dilemma regarding the optimum definition of the thickness and of the porosity of the cathode emissive coating; the object of the invention is to solve this dilemma.